The present invention relates to a fast breeder improved to exhibit a superior performance in the core region.
As well known to those skilled in the art, a fast breeder is a nuclear reactor in which neutrons generated by a fission in the reactor core are absorbed by a fertile material which in turn produces new fissile material. Thanks to this effect called "breeding", it is possible to make an efficient use of fuel.
The core of the nuclear reactor usually has a columnar construction consisting of a driver core region containing fissile material, and axial and radial blanket regions surrounding the driver core region and containing a fertile material. More specifically, the driver core region is loaded with a fuel which is usually an enriched uranium or an uranium enriched in plutonium, while the blanket regions are loaded with a fertile material such as natural uranium or depleted uranium. The fertile material captures the neutrons leaking from the reactor core to produce useful fissile material, i.e. plutonium.
In an ordinary core structure of fast breeder generally referred to as "homogeneous core" the core region is divided into a plurality of concentric parts: namely, an inner core region and an outer core region. In this type of core structure, the enhancement of fuel is higher in the outer core region than in the inner core region. The term "enhancement" is used to mean the ratio of the amount of fissionable material to the sum of the amount of fissionable material and the amount of fertile material.
In recent years, proposed is a core structure called "Parfait core" in which a flattened columnar internal blanket region of a constant thickness is disposed at the axially central portion of the core region of a fast breeder. For information, a reference shall be made to "Evaluation of the Parfait Blanket Concept for Fast Breeder Reactors", G. A. DUCAT et al., COO-2250-5, MITNE-157, Massachusetts Institute of Technology, January 1974.
Proposed also is a core construction for fast breeders in which an internal blanket region is disposed at an axially central portion of the core region as in the case of the Parfait core, wherein the axial thickness of the internal blanket region is reduced at the peripheral portion thereof. This type of core construction is disclosed in "A Fast Breeder Core With Internal Blanket", K. Inoue et al., American Nuclear Society Transactions, Vol. 33, pp. 862-863, 1979. In this core construction, the outer surface of the internal blanket region is held in contact with the inner surface of the radial blanket region.